Generally an electronically controlled sewing machine has a stepping motor for controlling the positions of a needle vertically reciprocating and laterally swingable and another stepping motor for controlling the feeding amounts of a fabric to be sewn.
So far, each of the stepping motors had a predetermined fixed current value applied thereto, and the fixed current value was required to be a comparatively higher than necessary. This is because the component parts of the sewing machine operatively connected between the stepping motor and the needle or a feed dog generally have dimensional errors which may be caused during the production though the errors are within a tolerable range. The production errors may be further caused in control circuits, stepping motors, stepping motor drive circuits, current setting circuits, etc. The loads and errors may be further variable by the change of environmental temperatures. Generally it is required that the set current value is sufficient enough to move the loads even if the components having the largest errors are combined.
With a current value comparatively higher than necessary, applied to the stepping motor according to the prior art, the stepping motor is provided with a sufficient torque to move a prescribed load, but has an electric current more than necessary flowing through the coil of the motor. As a result, a stepping motor drive device is under a heavy load and is heated up together with the stepping motor.